An anonymous security researcher known as Nightmare Eclipse has released a new Microsoft Defender zero-day exploit named “ShieldCrash” right after Microsoft rolled out its September 2026 Patch Tuesday security updates.

ShieldCrash is described as a bypass for the ShieldBreak Defender privilege escalation flaw patched on Thursday, which itself bypassed RoguePlanet, another Defender flaw disclosed in June and patched by Microsoft in July.

According to Nightmare Eclipse, the ShieldCrash proof-of-concept exploit lets attackers gain SYSTEM privileges on fully patched Windows 10, Windows 11, and Windows Server systems, but will not give them write access to the compromised systems.